The fact of terrestrial magnetism may be expressed by saying that
the space within and around the whole earth is filled by lines of
magnetic force, which we know nothing about until we suspend a
magnet so perfectly balanced that it may point in any direction
whatever. Then it turns and points in the direction of the lines
of force, which may thus be mapped out for all points of the
earth.

We commonly say that the pole of the needle points towards the
north. The poets tell us how the needle is true to the pole. Every
reader, however, is now familiar with the general fact of a
variation of the compass. On our eastern seaboard, and all the way
across the Atlantic, the north pointing of the compass varies so
far to the west that a ship going to Europe and making no
allowance for this deviation would find herself making more nearly
for the North Cape than for her destination. The "declination," as
it is termed in scientific language, varies from one region of the
earth to another. In some places it is towards the west, in others
towards the east.

The pointing of the needle in various regions of the world is
shown by means of magnetic maps. Such maps are published by the
United States Coast Survey, whose experts make a careful study of
the magnetic force all over the country. It is found that there is
a line running nearly north and south through the Middle States
along which there is no variation of the compass. To the east of
it the variation of the north pole of the magnet is west; to the
west of it, east. The most rapid changes in the pointing of the
needle are towards the northeast and northwest regions. When we
travel to the northeastern boundary of Maine the westerly